It is well known that most of the engineering structures, such as underground ducts, pipelines and storage tanks, do not possess the required impermeability characteristics, owing, in part, to the inadequate quality of the construction materials and, in part, to defects in the impermeability of pipe connections, or because of deterioration to the engineering structures upon the effect of ageing, traffic, etc. It is also well known that the repair of engineering structures, particularly underground ducts and pipelines, requires enormously high investments and labor, and in most cases the repairs are inadequate.
Hungarian Patent No. 153,975 describes a simple and rapid method for improving the strength and impermeability of soils and engineering structures. According to this method, water glass or a water glass-containing medium is applied into or onto the article to be treated, and then the water glass is exposed to the effect of hydrogen fluoride, silicon tetrafluoride and/or hydrogen silicofluoride. Water glass, when contacted with a gaseous fluoride, rapidly gellifies and completely plugs the leakages, cracks and cavities. When this method is utilized to render underground engineering structures (such as ducts or storage tanks) water-tight, it is an additional advantage that water glass which enters the soil through the cracks solidifies as well, improving thereby the embedding of the structure and strengthening the surrounding soil. Fluoride gases have the additional advantage that they improve the corrosion resistance of concrete and reinforced concrete elements.
Despite its numerous advantages, this method has had only a very narrow application in practice. The widespread application of this method is considerably restricted by the fact that hydrogen fluoride and silicon tetrafluoride are strongly poisonous, thus their use is prohibited in most countries by environmental protection requirements. It is a further disadvantage that the resulting silicic acid gel is not elastic, thus it cannot follow the movements of the article or soil treated. Since the swellability of silicic acid gels is inadequate, these gels cannot plug the new cracks formed in the gel upon movement.
In order to avoid the above disadvantages, methods have been developed in which aqueous acids are utilized to gellify water glass instead of the poisonous acidic fluoride gases, and, to ensure the required elasticity and swellability of the gel, synthetic organic polymers are formed in the gel structure in parallel with gel formation. Such methods are disclosed in Hungarian Patents Nos. 186,586 and 189,250, in the published Hungarian patent application No. 1095/84, as well as in the published international patent application No. PCT/HU-85/00027. A common feature of the techniques disclosed therein is that the gel is formed by admixing two liquids with one another: one of the liquids comprises water glass and organic polymer-forming components, such as polymerizable monomers or a linear polymer to be cross linked, which are compatible with water glass, whereas the other liquid is an aqueous solution of the acid required to gellify water glass and of the substances required to perform polymerization or cross-linking, such as catalyst, initiator, cross-linking agent, etc.
When the appropriate mixing of the two liquids is provided for, hydrogels of good quality, possessing appropriate strength, elasticity and swellability can be formed. However, with the so-called "filling-up" techniques generally applied in soil strengthening and in the water-tight plugging of the defects of engineering structures (particularly underground ducts, pipelines and tunnels) the appropriate mixing of the two liquids generally cannot be ensured throughout the whole section of the soil or engineering structure to be treated. The essence of filling-up technique is that first one of the two liquids, generally the water glass-containing solution, is applied onto the area to be treated e g. in such a way that the liquid is filled into the duct section to be repaired, and then, after an appropriate waiting time, the excess of the first liquid is optionally removed, and the second liquid, generally the aqueous solution comprising the acid and the substance required for polymerization, is introduced. Through the defect sites of uneven dimensions the liquids enter the soil at an uneven rate, and, as a consequence of the uneven pore and cavity structure of the soil, their movement rate in the soil is uneven, too.
There are considerable local variations between the mixing ratios of the two liquids This means that at certain sites gel cannot be formed at all in the first filling-up cycle, whereas at other sites, depending on the actual mixing ratio, considerable local variations in gel quality can be observed. Thus, e g. when a water glass solution is introduced first and then an aqueous solution which cannot form a gel per se upon neutralization (such as an aqueous solution comprising an acid other than hydrogen silicofluoride and additional non-gelling components, e.g. cross-linking agents, polymerization catalysts, etc.) is applied onto the water glass solution, no gel is formed from the portions of the acidic solution which are in excess or which cannot be mixed with the water glass solution. These portions of the acidic solution flow through the soil section, more or less impregnated with water glass, without utilization, cutting channel-like passages through the soil. These channel-like passages render the gel structure more or less open to water exfiltration; thus the water tightness of the gel formed in the first filling-up cycle is inadequate. A gel with the required water tightness can be obtained only by repeating the filling-up cycle two or more times, which decreases the efficiency of the process and involves a considerable increase in operation costs.
When each of the two liquids comprises a component which gellifies upon neutralization (such as one of the liquids comprises water glass and the other comprises a monomer or oligomer which polymerizes when contacting it with an alkali), uneven mixing of the two liquids (results in that) instead of obtaining a gel with homogenous microstructure in which the organic and mineral parts are amalgamated, a gel consisting of easily distinguishable discrete mineral (silicate) and organic (polymer) blocks, i.e. a gel with nonhomogeneous microstructure is formed. The strength and swelling characteristics of these organic and mineral blocks greatly differ from one another. These gels with nonhomogeneous microstructures combine essentially all the disadvantages of the completely mineral and completely organic gels in the mineral blocks the gel is non-elastic, rigid, and cracks upon soil movements, whereas in the organic blocks the gel is too soft and cannot withstand the damaging effects of higher strains.
Uneven mixing outlined above is a necessary concomitant of all filling up techniques which apply two liquids. In order to avoid the disadvantageous consequences of uneven mixing a pair of gel-forming liquids is required, in which
(1) a gel can be formed from both liquids upon a chemical reaction and,
(2) the properties of the gels formed separately from the two liquids are highly similar.
Furthermore, in order to decrease the rigidity of the gel and to ensure an appropriate swellability, a substance is required which does not polymerize and gellify per se, but which is able to be built into the gel structure wherein it exerts a plasticizing effect. In order to decrease the disadvantageous consequences of uneven mixing it is desirable that this substance should be able to reach by diffusion even those liquid portions which do not mix with one another.